HandlerMethodReturnValueHandler for replying directly to a
subscription. It is supported on methods annotated with
SubscribeMapping such that the return value is treated as a response to be
sent directly back on the session. This allows a client to implement
a request-response pattern and use it for example to obtain some data upon
initialization.

The value returned from the method is converted and turned into a
Message that is then enriched with the sessionId, subscriptionId, and
destination of the input message.

Note: this default behavior for interpreting the return
value from an @SubscribeMapping method can be overridden through use
of the SendTo or SendToUser annotations in which case a
message is prepared and sent to the broker instead.